Propylene demand in the petrochemical industry has grown substantially, largely due to its use as a precursor in the production of polypropylene for packaging materials and other commercial products. Other downstream uses of propylene include the manufacture of acrylonitrile, acrylic acid, acrolein, propylene oxide and glycols, plasticizer oxo alcohols, cumene, isopropyl alcohol, and acetone.
Propylene is typically produced during the steam cracking or pyrolysis of hydrocarbon feedstocks such as natural gas, petroleum liquids, and carbonaceous materials (e.g., coal, recycled plastics, and organic materials), to produce ethylene. Additional sources of propylene are byproducts of fluid catalytic cracking (FCC) and reside fluid catalytic cracking (RFCC), normally targeting gasoline production. FCC is described, for example, in U.S. Pat. No. 4,288,688 and elsewhere. A mixed, olefinic C3/C4 hydrocarbon byproduct stream of FCC may be purified in propylene to polymer grade specifications by the separation of C4 hydrocarbons, propane, ethane, and other compounds.
More recently, the desire for propylene and other light olefins from alternative, non-petroleum based feeds has led to the use of oxygenates such as alcohols and, more particularly, methanol, ethanol, and higher alcohols or their derivatives. Methanol, in particular, is useful in a methanol-to-olefin (MTO) conversion process described, for example, in U.S. Pat. No. 5,914,433. The yield of light olefins from such a process may be improved using olefin cracking to convert some or all of the C4+ product of MTO in an olefin cracking reactor, as described in U.S. Pat. No. 7,268,265. Other processes for the targeted production of light olefins involve high severity catalytic cracking of naphtha and other hydrocarbon fractions. A catalytic naphtha cracking process of commercial importance is described in U.S. Pat. No. 6,867,341.
Paraffin dehydrogenation represents yet another dedicated route to light olefins and is described in U.S. Pat. No. 3,978,150 and elsewhere. However, the significant capital cost of a propane dehydrogenation plant is normally justified only in cases of large-scale propylene production units. The substantial supply of propane feedstock required to maintain this capacity is typically available from propane-rich liquefied petroleum gas (LPG) streams from gas plant sources.
From any of the foregoing processes, the propylene may be used to produce polypropylene. In order to produce polypropylene, the propylene must be in a stream typically has a purity of at least 99.5% by volume. However, some producers may not require such a high level of purity. For example, a purity of at least about 97% by volume (e.g., in the range from about 97% to about 99% by volume) or at least about 98% by volume (e.g., in the range from about 98% to about 99% by volume) may be acceptable for a non-polymer technology such as acrylonitrile production.
However, some polypropylene producers may be able to produce polypropylene from propylene having a purity lower than 99.5% by volume.
Therefore, it would be desirable to have one or more processes for producing polypropylene from a propylene stream having a purity below 99.5% by volume. It would be also desirable if such processes were economically viable.